An electric arc, in particular in the case of a three-phase alternating current supply, supplies energy in the form of arcs through three graphite electrodes to a melting furnace. A melting furnace of this sort is, for example, an electric arc furnace, or EAF. Usually for this purpose the medium voltage or high voltage is transformed down to a low voltage and supplied to the electrodes with the aid of a furnace transformer. The electrodes can be moved mechanically upwards or downwards in order to ignite the arc and then, by means of the spacing, to adjust the arc voltage and the current, and thus the power coupled in. Since in each phase the current becomes zero twice in each period, there is a risk that the arc extinguishes if a current is not developed in the opposite direction with a sufficiently steep current rise before the plasma that is present cools down too far.
Conventionally a sufficiently large inductor in the circuit ensures that the current persists for long enough that, at the current zero-transition, the voltage is already high enough to maintain the flow of current. This has the result that the arc furnace can only be operated up to a certain power factor cos φ, and has a high reactive power requirement. Typical values for the power factor lie around about 0.83 and this, with a transformer having an apparent power of 100 MVA yields a real power input of 83 MW and a reactive power of 56 MVAr. The electrical operating equipment can therefore not be optimally utilized. Conventionally, a regulation of an arc furnace is in the range of seconds, as a consequence of the mechanical movement of the electrodes.